This invention relates to a circuit for driving a voltage controlled oscillator (VCO), and more particularly to a circuit for switching a VCO between two preset variable frequencies and simultaneously imposing frequency modulation about the selected center frequency. When it is desired that the VCO generate a single preselected frequency, it is a simple matter to control the output of the voltage controlled oscillator by means of a feedback path which senses the VCO output, determines the frequency difference between the VCO output and the desired VCO output and generates therefrom a controlling DC voltage for changing the output frequency of the VCO. However, no known circuit exists which will switch a voltage controlled RF oscillator between two preset variable frequencies and simultaneously impose frequency modulation about the selected center frequency.